This invention relates to optical recording media having layered structures and, in particular, to layered optical recording media having improved stability and weatherability.
Optical recording media that permit reading, writing and erasing are of recent interest. In particular, magneto-optic recording media utilizing the magneto-optic effect have recently been put into practical use as erasable-rewritable optical recording media. One of the problems that has been encountered with such media is that the magneto-optic recording layers often have inferior durability because they contain rare earth transition metals. It is for this reason that the recording layer is generally sandwiched with transparent dielectric layers prior to adhesion of the opposing substrates.
A conventional magneto-optic recording medium 10 in which the substrates are adhered together is shown in FIG. 1. A polycarbonate substrate 11 having a thickness up to about 1.2 mm is formed with grooves having a groove pitch of up to about 1.6 .mu.m, a groove width of up to about 0.8 .mu.m and a groove depth of up to about 700 .ANG. on one surface. A silicon nitride dielectric layer 12 having a thickness up to about 1000 .ANG. is formed on the grooved side of grooved polycarbonate substrate 11 and a magneto-optic recording layer 13 having a thickness up to about 450 .ANG. is formed on silicon nitride layer 12. The magneto-optic recording layer can be, for example, terbium-iron-cobalt (TbFeCo) or neodymium-dysprosium-iron-cobalt (NdDyFeCo). A silicon nitride dielectric layer 14 having a thickness up to about 1000 .ANG. is formed on magneto-optic recording layer 13. Substrate 11, dielectric layer 12, magneto-optic recording layer 13 and dielectric layer 14 together form an optical transmission substrate 17. A smooth polycarbonate substrate 16 is disposed on and secured to dielectric layer 14 of optical transmission substrate 17 by an ultraviolet (UV) curing resin layer 15.
One of the problems that arises with this type of recording medium is that gas bubbles are mixed with the UV curing resin layer when the substrates are adhered under vacuum. It is believed that these bubbles result from evaporation of a volatile substance contained in the substrate. Bubbles in the adhesive layer prevent the reading and writing of correct information from or to the recording layer because the bubbles cause the dielectric layer to crack. This causes deterioration in the bit error rate.
Furthermore, after joining the substrates together, the internal and circumferential areas of the recording medium, which is generally in the form of a disk, are exposed. When an accelerated aging test is conducted at a temperature of 60.degree. C. and 90% relative humidity, deterioration of the recording layer from the internal and circumferential areas occurs within about 100 hours since the adhesive forces generated by the ultraviolet curing adhesive are weak. This causes the adhesive interface to peel when the adhered substrates are tested for aging in a high temperature, high humidity chamber. Additionally, the adhesive interface deteriorates when the adhesive is removed from either the circumferential or internal area of a disk by mechanical processes.
Accordingly, it is desirable to provide a recording medium having improved adhesion between the substrates.